The present invention generally relates to electro-mechanical assemblies, such as probe cards having electrical contacts for testing integrated circuits.
Electro-mechanical assemblies, such as probe cards, can be used in testing a die, e.g. integrated circuit devices, typically on semiconductor wafers. Such probe card is used in connection with a device known as a tester, wherein the probe card is electronically connected to the tester device, and in turn the probe card is also in electrical contact with the integrated circuit to be tested (either on a wafer or in singulated form) (as referred to herein an integrated circuit to be tested in any form is referred to as a device under test (DUT)).
Typically a wafer to be tested is loaded into a prober securing it to a movable chuck. During the testing process, the chuck moves the wafer into electrical contact with the probe card. This contact occurs between a plurality of probes on the probe card, which can be in the form of spring-type contacts, and a plurality of discrete connection pads (bond or test pads) on the DUTs. Several different types of electrical contacts are known and used on probe cards, including without limitation needle contacts, cobra-style contacts, spring contacts, and the like. In this manner, the DUTs can be tested and exercised, prior to being singulated from the wafer, or final packaging.
For effective contact between the probes of the probe card and the bond pads of the dies, the distance between the probe card and the wafer should be properly maintained. If portions of the DUT and the probes are too far apart contact can be lost, if they are too close together then damage may occur to the probes or the DUTs, or, for example, the probe can move from a desired location on the DUT.
The distance between the DUT and the probe card may change as the testing procedure proceeds, especially as the temperatures of the probe card and the wafer change during the testing process. In many instances, the wafer being tested may be heated or cooled during the testing process. When a wafer having a temperature different than that of the probe card is moved under the card, the card face nearest the wafer begins to change temperature. Probe cards are typically built of layers of different materials and are usually poor heat conductors in a direction normal to the face of the card. As a result, a thermal gradient across the thickness of the probe card can appear rapidly. As a result of this uneven expansion, the probe card can begin to sag, decreasing the distance between the probe card and the wafer. The opposite phenomenon may occur when a wafer is cooler than the ambient temperature of the tester is placed near the probe card. As the face of the probe card nearest the wafer cools and contracts faster than the face farthest from the wafer, the probe card begins to bow away from the wafer disrupting electrical contact between the wafer and the probe card.
Moreover, various parts of the probe card may have different coefficients of thermal expansion. As a result, they may expand or contract at different rates as the temperature of the environment in which the probe card and the wafer are placed for testing changes. This would also cause the probe card to deform, thereby moving the probes away from the pads on the wafer, and causing the test to fail.
Thus, what is needed is an apparatus for adjusting the thermally induced movement of probe cards, such that the probes can remain in proper contact with the pads of DUTs on wafers.